Numerous different ion exchange resins have been proposed for the selective extraction of gold from leach solutions. Weak-base resins were generally considered to be the most advantageous in view of the anticipated ease of elution with aqueous sodium hydroxide. However, such weak-base resins were accompanied by various disadvantages.
Work has been conducted in addition on commercially available strong-base resins, such resins having a higher capacity than do weak-base resins in the typical gold-cyanide liquor. Also, the chemistry involved in adsorption of ions onto strong-base resins is less complex than in the case of weak-base resins. This is because the charge on the resin is fixed and the effect of pH can generally be disregarded.
In spite of these advantages, the selectivity of such commercially available strong-base resin towards gold has as far as applicant is aware, been inadequate.
Test work has been carried out on certain experimental strong base resins having various different functional amine groups. (See article entitled "The Extraction of Metals from alkaline cyanide solutions by basic ion exchange materials" by A. A. Buggs et al - Published by Department of Scientific and Industrial Research, National Chemical Laboratory, Teddington, Middlesex 1963). These resins, however, whilst having a satisfactory loading and selectivity towards gold, exhibited poor elution characteristics and were accordingly unsuitable for commercial exploitation. Effective elution could only be achieved when the resin was further modified by the inclusion of weak-base groups and furthermore a methanolic solution of thiourea or thiocyanate had to be used as eluant. Aqueous solutions of thiourea, which are better suited to commercial application required about 40 bed volumes of eluant to effectively remove adsorbed gold from a structure modified with weak-base groups. This volume of eluant is undesirable in a commercial operation.
It has now surprisingly been found that such drawbacks are satisfactorily alleviated by properly selecting the resin matrix.
It is, accordingly, the object of this invention to provide a strong-base ion exchange resin which exhibits good selectivity towards gold and, generally speaking, advantageous properties for the extraction and elution of gold.